The demonstration of thick filaments in smooth muscle of any organ is of fairly recent origin and the role of adequate fixation and consequent artefacts may contribute to the confusion on whether they exist or not. Results of our preliminary study suggest that the ovarian steroids may control the assembly and disassembly of thick filaments in cat myometrium. We observed both thick and thin filaments in myometrium obtained from animals in estrus, or in ovariectomized animals treated with estradiol. However, only thin filaments were observed in myometrial tissue removed from animals with elevated levels or serum progesterone. This alteration in the structure of thick filaments may be involved in the mechanism by which these hormones alter the contractile lactivity of myometrium. In the proposed study we will attempt to document whether thick filaments are present in human and cat myometrium and if their presence or absence is dependent on the levels of ovarian steroids. We will do this by using the morpholigical techniques of electron microscopy and image enhancement of the thick filaments by the immuno gold staining technique, and the biochemical techniques of disc gel electrophoresis and steroid receptor determination. These data will be correlated with the serum levels of estradiol and progerstrone as determined by radioimmunoassay. Our study will critically evaluate the presence or absence of thick filaments in the pregnants uterus and what effect, if any, fetal development has on myometrial structure. Special emphasis will also be placed on the question of whether or not thick filaments are present at the time of, or immediately prior to parturition, since any morphological alteration in the contractile apparatus at this time may have profound effects on the mechanism of pregnancy termination.